Wheel is not included in Python 3

The wheel package is included in Python 2 but is not in Python 3:

```
docker run --rm python:2 bash -c "pip list | grep wheel"
wheel (0.29.0)

docker run --rm python bash -c "pip list | grep wheel"
```

This results in pip not building wheels in Python 3, while it does build them in Python 2.
